Coal-fired power facilities represent a significant source of airborne mercury emissions in the United States. Some level of mercury emission control is achieved by use of air pollution control devices, such as those utilizing activated carbon injection. However, carbon injection requires specialized adsorbents designed to capture mercury from the facility flue gas stream. The level of control achieved is further influenced by a variety of factors including the type of coal used, fly ash composition, and the existing air pollution technology at any given coal-fired power facility, among other factors that can be considered in determining appropriate emission control for mercury removal. It is anticipated that many existing air pollution devices will likely not be able to achieve future mercury emission limitations.
Specialized adsorbents are commonly prepared by impregnating an adsorbent with an additive that possesses chemical properties the adsorbent itself does not have. The impregnation process can be time consuming and expensive. Some materials used in the process are gases or fuming liquids which require special storage and handling. They can also require special worker protection since the materials can present serious respiratory hazards.
Some prior adsorbents have the further disadvantage that the adsorbent cannot be impregnated effectively because the amount of chemical additive needed to make it effective would necessarily block the pores of the adsorbent thereby precluding adsorption of the mercury. Current methods of impregnating the adsorbent have also been known to lead to uneven distribution of additive or catalyst loadings. For example if the material were sprayed onto the adsorbent, depending upon the process selected, a portion of the adsorbent produced could have high levels of additive while other portions could have little or no additive.
Thus, there is a need for an improved adsorbent and for a process for making an adsorbent for use in mercury removal. There is also a need for an adsorbent having an appropriate level of uniformity. Additionally, it is desirable to identify a process that can produce a product with acceptable mercury removal performance.